This application incorporates by reference Taiwanese application Serial No. 88114438, filed Aug. 24, 1999.
1. Field of the Invention
The invention relates in general to the clock generating apparatus and method for generating clock signals of different frequency, and more particularly to the clock generating apparatus and method that utilize low frequency oscillation to generate clock signals of different frequency which are in phase alignment.
2. Description of the Related Art
Due to the fast development of computer technologies, the internal frequency of the central processing unit, CPU, has been up to hundreds of MHz nowadays rather than several MHz before. The internal frequency of the CPU is gained by multiplying the external frequency of the CPU by a multiple. For example, a CPU""s internal frequency of 266 MHz can be gained by multiplying an external frequency of 66 MHz and a multiple of 3.5. Therefore, in order to achieve higher CPU internal frequency, the motherboard of the computer should be provided with higher external frequency.
However, for the motherboards nowadays, the external frequency of the CPU may not be higher than the frequency of the other components such as memory. In general, the external frequency of the CPU may be 133 MHz, 100 MHz, or 66 MHz. For example, the frequency of memory, dynamic random access memory, DRAM, may be 133 MHz, 100 MHz, 66 MHz, or 50 MHz. And the frequency of peripheral component interconnect, PCI, may be 66 MHz or 33 MHz.
Therefore, if the motherboard can provide several frequency modes of the CPU""s external frequency versus the frequency of memory, the motherboard products will be more attractive to consumers. Seven common frequency modes of the CPU""s external frequency versus the frequency of the memory are as follows, 133/133, 133/100, 100/100, 100/66, 66/100, and 66/66.
As a result, a motherboard should be able to provide several kinds of clock signals to meet the needs of different frequency modes. Besides, all of the provided clock signals should be pseudo synchronous. Otherwise, when any one of the clock signals is transformed into another clock signal, the signals might get lost or be duplicated. To make the clock signals pseudo synchronous, these clock signals should be in phase alignment. That is to say, the first phase signal of all clock signals should be all at the high level or at the low level. The meaning of the phase alignment is illustrated in FIG. 1. As shown in FIG. 1, two clock signals CLK-A and CLK-Bare taken as an examples for illustration and the frequency ratio of the two is about 3:2. Suppose the frequency of CLK-A and CLK-B are 100 MHz and 66 MHz, respectively. The clock signal CLK-A is provided with three phase signals PH1A, PH2A, and PH3A. Similarly, the clock signal CLK-B is provided with two phase signals PH1B, and PH2B. When the clock signal is enabled, it is at high level. The three phase signals PH1A, PH2A, and PH3A of the clock signal CLK-A are in turn enabled. That is to say, the phase signal PH1A is enabled in the first period of the clock signal CLK-A. The phase signal PH2A is enabled in the second period of the clock signal CLK-A. And, the phase signal PH3A is enabled in the third period of the clock signal CLK-A. Likewise, the two phase signals PH1B and PH2B of the clock signal CLK-B are in turn enabled.
Referring to FIG. 1, phase alignment of the clock signals CLK-A and CLK-B means that the first phase signals PH1A and PH1B of the two clock signals CLK-A and CLK-B are enabled at the same time. Namely, when the first phase signals of a number of clock signals are enabled at the same time, these clock signals are in phase alignment.
It is important to achieve the pseudo synchrony of clock signals of different frequency because the transformation between the clock signals of different frequency is correct only when the clock signals are in pseudo synchrony. If they are not in pseudo synchrony, errors such as signal loss or multiple transformations may occur. The pseudo synchrony of different clock signals can be achieved only when the phase alignment of the clock signals is achieved.
Conventionally, a phase-locked loop, PLL, is utilized to generate clock signals of different frequency. Referring to FIG. 2, the block diagram of a conventional PLL is illustrated. As shown in FIG. 2, the PLL 200 includes a phase detector 210, a low-pass filter 220, and a voltage-controlled oscillator, VCO, 230.
Referring to FIG. 2, the divider 202 is utilized to divide the input signal FIN by D and output a base band signal FIN/D, D is an integer. In the following paragraphs, the above-mentioned divider is called Divide-by-D divider. Therefore, a divider dividing the input signal by N is called Divide-by-N divider. The base band signal FIN/D is transmitted through the phase detector 210, the low-pass filter 220, and the VCO 230. After the PLL 200 is locked, the VCO 230 outputs an output signal FOUT. Then, the output signal FOUT is fed back to the phase detector 210 by a Divide-by-N divider 204. That is to say, the Divide-by-N divider 204 outputs a base band signal FOUT/N to the phase detector 210. Therefore, The two base band signals FIN/D and FOUT/N have the following relationship:
FOUT/N=FIN/D.
Then, FOUT=(N/D)xc3x97FIN. Therefore, by appropriately dividing the output signal of the PLL 200, various clock signals can be obtained. And the phases of clock signals of different frequency can be in phase alignment through the use of a reset signal (not shown in the FIG.).
As mentioned above, with the properly chosen input signal and dividers, clock signals of different frequency can be obtained. For example, when the input signal FIN is 100 MHz, D=3, and N=12, the output signal FOUT=(N/D)xc3x97FIN=(12/3)xc3x97100 MHz=400 MHz. Then, a clock signal of 133 MHz can be obtained by dividing the output signal FOUT by 3.
Generally speaking, the output signal FOUT and the input signal FIN are multiples of a basic clock signal. For example, 133 MHz, 100 MHz, and 66 MHz are four times, triple, and twice of 33 MHz, respectively.
Conventionally, the output signal FOUT should be higher than 400 MHz to meet the needs of generating different clock signals. For example, 133 MHz is obtained by dividing the output signal of 400 MHz by 3. In the same way, 100 MHz and 66 MHz are obtained by dividing 400 MHz by 4 and 6, respectively. However, the effect of the oscillation of the VCO produced by the present semiconductor techniques is not steady enough. Therefore, errors may occur.
To prevent the VCO of the PLL from oscillating in the high frequency, two or more PLLs are utilized to generate output signals of different frequency conventionally. However, utilizing a number of PLLs to generate clock signals of different frequency raises other problem. That is, the clock signals generated by these PLLs are not in phase alignment. Therefore, errors, such as signal loss, may occur while the clock signals are transmitted.
It is therefore an object of the invention to provide a clock generating apparatus and method, wherein clock signals of different frequency can be generated by appropriately combining the reset signal, dividers, and PLLs. Besides, the clock signals generated are all in phase alignment with the main clock signal. Moreover, the PLLs are not oscillated at high frequency, which means that the clock generating apparatus is more stable.
The invention achieves the above-identified objects by providing a clock generating apparatus for receiving a main clock signal and a reset signal and outputting a number of clock signals. The main clock signal is provided with m phase signals, m is an integer. The m phase signals are enabled in turn during the m periods of the main clock signals. Each of the m phase signals is enabled in one period of the main clock signal. Besides, the reset signal is enabled when the mth phase signal of the main clock signal is enabled. The clock generating apparatus includes five dividers and the first phase-locked loop (PLL). The first divider is used for receiving the main clock signal and the reset signal. After the first divider is reset by the reset signal, the first divider divides the main clock signal and outputs a reference clock signal when the first phase signal of the main clock signal is enabled. The first PLL, is used for receiving the reference clock signal and the first feedback clock signal. The first PLL outputs the first clock signal after the reference clock signal and the first feedback clock signal are phase-locked. The second dividers used for receiving the reset signal and the first clock signal. After the second divider is reset by the reset signal, the second divider divides the first clock signal and outputs the third clock signal when the first phase signal of the main clock signal is enabled. The third divider is used for receiving the reset signal and the second clock signal. After the third divider is reset by the reset signal, the third divider divides the second clock signal and outputs the forth clock signal when the first phase signal of the main clock signal is enabled. The forth divider is used for receiving the reset signal and the third clock signal. After the forth divider is reset by the reset signal, the forth divider divides the third clock signal and outputs the first feedback clock signal when the first phase signal of the main clock signal is enabled. The fifth divider is used for receiving the reset signal and the forth clock signal. After the fifth divider is reset by the reset signal, the fifth divider divides the forth clock signal and outputs the second feedback clock signal when the first phase signal of the main clock signal is enabled. Furthermore, the frequency of the first feedback clock signal and the second feedback clock signal are the same as that of the reference clock signal. Besides, the third clock signal, the forth clock signal, and the main clock signal are in phase alignment.
The invention achieves the above-identified objects by providing a clock generating method for receiving a main clock signal and a reset signal and outputting a number of clock signals. The clock generating method includes the following steps. First, m phase signals are generated, m is an integer. The period of each of the m phase signals is m times of the period of the main clock signal. Besides, each of the m phase signals is enabled during one period of the main clock signal. Furthermore, the reset signal is enabled when the m phase signal is enabled.
Second, the main clock signal and the reset signal are received by the first divider. After the first divider is reset by the reset signal, the first divider divides the main clock signal and outputs a reference clock signal when the first phase signal is enabled.
Then, the reference clock signal and the first feedback signal are received by the first phase-locked loop (PLL). After the reference clock signal and the first feedback clock signal are phase-locked, the first PLL outputs the first clock signal.
Next, the reference clock signal and the second feedback signal are received by the second PLL. After the reference clock signal and the second feedback clock signal are phase-locked, the second PLL outputs the second clock signal.
Then, the reset signal and the first clock signal are received by the second divider. After the second divider is reset by the reset signal, the second divider divides the first clock signal and outputs the first feedback clock signal when the first phase signal is enabled.
Next, the reset signal and the second clock signal are received by the third divider. After the third divider is reset by the reset signal, the third divider divides the second clock signal and outputs the second feedback clock signal when the first phase signal is enabled.
Then, the reset signal and the third clock signal are received by the forth divider. After the forth divider is reset by the reset signal, the forth divider divides the third clock signal and outputs the third clock signal when the first phase signal is enabled.
Finally, the reset signal and the forth clock signal are received by the fifth divider. After the fifth divider is reset by the reset signal, the fifth divider divides the forth clock signal and outputs the forth clock signal when the first phase signal is enabled.
The frequency of the first feedback clock signal and the second feedback clock signal are the same as that of the reference clock signal. Furthermore, the third clock signal, the forth clock signal, and the main clock signal are in phase alignment.